The present invention relates to field effect transistors having a non-volatile memory effect of the metal-insulant-semiconductor (MIS) type in which the semiconductor is a compound of group III-V and the insulant a double dielectric layer.
MIS transistors on silicon are well known. In the case of silicon several types of insulants are known, whose quality refers to the following different criteria:
low density of defects called interface states between the insulant and the semiconductor, PA1 low density of electrical charges in the insulant, PA1 good voltage response.
According to these criteria MIS on silicon and particularly MOS, i.e. MIS in which the insulant is an oxide are of excellent quality. Thus, silicon MOS with SiO.sub.2 as the insulant are very frequently used, as are MNOS, which has a more complex structure: metal-silicon nitride (Si.sub.3 N.sub.4)-oxide SiO.sub.2 -semiconductor.
The silicon MOS operates with a charge reversal operating mode, i.e. the metal grid is polarized in such a way that a charge of the opposite type to that of the semiconductor is formed at the insulant-semiconductor interface. For example if the semiconductor is of the p type, the grid is positively polarized and a negative charge constituted by electrons forms in the semiconductor at the interface with the insulant. This charge constitutes the N-type channel of the field effect transistor.
In MNOS transistors the creation of charges in the transistor channel may involve various more complex phenomena permitting their use in logic circuits with a non-volatile memory. These phenomena can be briefly described as follows, using as an example an N-channel transistor.
In the case of low positive grid polarizations the MNOS transistor functions in accordance with the same principle as the MOS transistor described. For higher positive grid polarizations electrons can traverse the SiO.sub.2 oxide layer by a tunnel effect if the layer has a limited thickness. The electrons pass through the oxide in the semiconductor-oxide direction and are fixed at the oxide-nitride interface due to the existence of traps at this point. When the polarization breaks down the electrons trapped at the oxide-nitride interface induce a positive charge in the semiconductor close to the interface with the oxide. Due to this positive charge it is necessary for the purpose of forming a negative charge channel to polarize the grid more strongly than was necessary prior to the trapping of the electrons at the oxide-nitride interface. Thus, there is a non-volatile memory effect of the high polarization creating the tunnel effect.
However, despite these very interesting possibilities MOS and MNOS transistors on silicon have very limited performance levels. These limitations are due to the limited mobility of electrons in silicon compared with the mobility in GaAs or other compounds of the III-V group.
Field effect transistors on GaAs are essentially limited to Schottky grid transistors, called MESFET (metal Schottky field effect transistor). In logic circuits MESFET's with a normally open channel require complicated circuitry and consume a large amount of energy. However, MESFET's with a normally closed channel are extremely complicated from the construction standpoint. In all cases MESFET's cannot be used as a non-volatile memory in the same way as MNOS. The prior art GaAs MESFET's are often of poor quality because the GaAs-insulant interface has a high density of interface states.
French Patent Application Nos. 79 22 301 and 79 26 611 of the applicant company describe MISFET transistors having two insulant layers, one constituted by Al.sub.x Ga.sub.1-x As and the other by a broad forbidden band insulant such as SiO.sub.2, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3. The use of Al.sub.x Ga.sub.1-x As as the first dielectric layer makes it possible to reduce the density of interface states. However, these MISFET's cannot be used as a non-volatile memory.